title: Impact and costs of incentives to reduce attrition in online trials: two randomized controlled trials.
abstract: BACKGROUND
  Attrition from follow-up is a major methodological challenge in randomized trials. Incentives are known to improve response rates in cross-sectional postal and online surveys, yet few studies have investigated whether they can reduce attrition from follow-up in online trials, which are particularly vulnerable to low follow-up rates.
  
  
  OBJECTIVE
  Our objective was to determine the impact of incentives on follow-up rates in an online trial.
  
  
  METHODS
  Two randomized controlled trials were embedded in a large online trial of a Web-based intervention to reduce alcohol consumption (the Down Your Drink randomized controlled trial, DYD-RCT). Participants were those in the DYD pilot trial eligible for 3-month follow-up (study 1) and those eligible for 12-month follow-up in the DYD main trial (study 2). Participants in both studies were randomly allocated to receive an offer of an incentive or to receive no offer of an incentive. In study 1, participants in the incentive arm were randomly offered a £5 Amazon.co.uk gift voucher, a £5 charity donation to Cancer Research UK, or entry in a prize draw for £250. In study 2, participants in the incentive arm were offered a £10 Amazon.co.uk gift voucher. The primary outcome was the proportion of participants who completed follow-up questionnaires in the incentive arm(s) compared with the no incentive arm.
  
  
  RESULTS
  In study 1 (n = 1226), there was no significant difference in response rates between those participants offered an incentive (175/615, 29%) and those with no offer (162/611, 27%) (difference = 2%, 95% confidence interval [CI] -3% to 7%). There was no significant difference in response rates among the three different incentives offered. In study 2 (n = 2591), response rates were 9% higher in the group offered an incentive (476/1296, 37%) than in the group not offered an incentive (364/1295, 28%) (difference = 9%, 95% CI 5% to 12%, P < .001). The incremental cost per extra successful follow-up in the incentive arm was £110 in study 1 and £52 in study 2.
  
  
  CONCLUSIONS
  Whereas an offer of a £10 Amazon.co.uk gift voucher can increase follow-up rates in online trials, an offer of a lower incentive may not. The marginal costs involved require careful consideration.
  
  
  BACKGROUND
  ISRCTN31070347; http://www.controlled-trials.com/ISRCTN31070347 (Archived by WebCite at http://www.webcitation.org/5wgr5pl3s).
authors.affiliation: E-health Unit, Research Department of Primary Care and Population Health, University College London, London, United Kingdom. z.khadjesari@ucl.ac.uk
authors.forename: Zarnie
authors.initials: Z
authors.lastname: Khadjesari
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Murray
authors.forename: Eleftheria
authors.initials: E
authors.lastname: Kalaitzaki
authors.forename: Ian R
authors.initials: IR
authors.lastname: White
authors.forename: Jim
authors.initials: J
authors.lastname: McCambridge
authors.forename: Simon G
authors.initials: SG
authors.lastname: Thompson
authors.forename: Paul
authors.initials: P
authors.lastname: Wallace
authors.forename: Christine
authors.initials: C
authors.lastname: Godfrey
mesh_terms: D000328:Adult; D000428:Alcohol Drinking; D003365:Costs and Cost Analysis; D034742:Electronic Mail; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D020407:Internet; D008297:Male; D008875:Middle Aged; D009042:Motivation; D010865:Pilot Projects; D013997:Time Factors